As the problems relating to the design and fabrication of broadband optical fibers are being successfully resolved, attention is now shifting to the investigation of methods for exploiting their great potential. This involves the ability to modulate a property of optical waves with an information signal. The wave property can be its intensity, phase, frequency, or polarization. While both analog and digital formats are suitable for modulating the optical carrier in a fiber system, digital modulation provides some noise immunity and is ideally suited to fiber transmission where the available bandwidth is large.
Intensity modulation is relatively simple to implement with presently available optical sources such as electroluminescent light-emitting diodes (LEDs) and injection lasers. These sources can be modulated directly by varying their driving currents at rates up to a few gigahertz. Pulse widths in the nanosecond range are realized by this technique. Other optical sources, such as the neodymium laser, that cannot be directly modulated at high frequencies require separate, external modulators. One such device, illustrated in U.S. Pat. No. 4,005,927, comprises an optical carrier waveguide and an electrical waveguide for an electrical drive signal. The parameters of the two waveguiding structures are chosen such that the phase velocities of the optical energy and the electrical drive signal are approximately equal. While this structure phase modulates the optical signal, it can be implemented as an amplitude modulator by the addition of appropriate external polarizers.
Also disclosed in the above-identified patent is an optical switch comprising an optical directional coupler and a travelling wave electrical drive circuit. In operation, the interaction of the electrical drive signal and the optical signal causes the optical signal in one of the optical waveguides to be transferred over into the other optical waveguide.
As perceived by the patentee, one advantage of the above-described structures resides in their increased efficiency over a much greater drive signal bandwidth. Thus, it would appear that quasi-linear operation is contemplated in which the bandwidth of the drive signal is quite similar to the bandwidth of the modulation impressed upon the optical signal. Such an arrangement, however, places a severe strain upon the drive signal circuit and drive source particularly if one wishes to generate very narrow optical pulses. With today's technology, generators using the best semiconductor devices are capable of generating 10 to 100 picosecond pulses. Thus, this prior art technique cannot be used at present to generate subpicosecond pulses.